Problem: A $29$ -meter ladder is sliding down a vertical wall so the distance between the top of the ladder and the ground is decreasing at $7$ meters per minute. At a certain instant, the bottom of the ladder is $21$ meters from the wall. What is the rate of change of the distance between the bottom of the ladder and the wall at that instant (in meters per minute)? Choose 1 answer: Choose 1 answer: (Choice A) A $\dfrac{147}{20}$ (Choice B) B $\dfrac{20}{3}$ (Choice C) C $20$ (Choice D) D $7$
Answer: Setting up the math Let... $a(t)$ denote the distance between the top of the ladder and the ground at time $t$, $b(t)$ denote the distance between the bottom of the ladder and the wall at time $t$, and $c$ denote the length of the ladder (which is always $29$ meters). $a(t)$ $b(t)$ $c$ We are given that $c=29$ and $a'(t)=-7$ (notice that $a'$ is negative). We are also given that $b(t_0)=21$ for a specific time $t_0$. We want to find $b'(t_0)$. Relating the measures The measures relate to each other through the Pythagorean theorem: $\begin{aligned} [a(t)]^2+[b(t)]^2&=c^2 \\\\\\ [a(t)]^2+[b(t)]^2&=29^2 \end{aligned}$ We can differentiate both sides to find an expression for $b'(t)$ : $b'(t)=-\dfrac{a(t)a'(t)}{b(t)}$ Using the information to solve In order to find $b'(t_0)$ we need to find $a(t_0)$. Using the Pythagorean theorem and the fact that $b(t_0)=21$ and $c=29$, we can find that $a(t_0)=20$. Let's plug ${a(t_0)}={20}$, ${a'(t_0)}={-7}$, and ${b(t_0)}={21}$ into the expression for $b'(t_0)$ : $\begin{aligned} b'(t_0)&=-\dfrac{{a(t_0)}{a'(t_0)}}{{b(t_0)}} \\\\ &=-\dfrac{({20})({-7})}{({21})} \\\\ &=\dfrac{20}{3} \end{aligned}$ In conclusion, the rate of change of the distance between the bottom of the ladder and the wall at that instant is $\dfrac{20}{3}$ meters per minute. Since the rate of change is positive, we know that the distance is increasing.